High-performance engineering plastics materials made of thermoplastic resins reinforced by carbon fibers have recently attracted researchers' attention and the demand for them has increased rapidly. However, the carbon fiber does not have active groups on the surface, so it is inherently inactive and has poor adhesion to a thermoplastic resin. A simple blend of the carbon fiber and the thermoplastic resin has improved mechanical properties over the matrix resin, but the improvement is not great enough to fully utilize the characteristics of the carbon fiber. The poor adhesion between the carbon fiber and the thermoplastic resin has adverse effects on the impact strength of the reinforced resin.
The common method of blending a carbon fiber with a thermoplastic resin is to supply a fiber of a given length (e.g. 10-3 mm) and resin pellets or a resin powder to an extruder and mix the two in a molten state. Unless the fiber has high gathering properties (adhering property of fibers to each other to keep the fibers in the form of a bundle), the fibers "open" (fibers of a part or all of a bundle separate from each other) due to the friction with the resin, and the fluffy fiber floats in the hopper to cause only the resin to be fed into the extruder, and as a natural consequence, it is difficult to have a steady flow of a uniform resin composition. To avoid this problem, the fiber must be given gathering property high enough to prevent opening. One way to increase the gathering property of the carbon fiber is to coat it with a thermoplastic resin the same as the matrix resin. In this case, the coating resin is usually dissolved in a solvent. For example, formic acid or phenol could be used as solvents for polyamide resins, and chlorinated hydrocarbon solvents for polycarbonate, polysulfone, and polyether sulfone. However, these solvents deteriorate the environment in the treating steps, and there are no harmless solvents for these resins. In this respect, it is not desirable to increase the gathering property of the carbon fiber with such resins. To avoid the difficulties mentioned above, a milled fiber not longer than 1 mm could be used, but by the shearing action of the screw in the extruder, the fiber is broken down to shorter lengths, and the average length of the fiber present in the final molding is 0.08 mm or less, which hardly contributes to improving the characteristics of the molding. Therefore, the milled fiber is not suitable for the purpose of obtaining a molding of high strength and modulus of elasticity.
Carbon fibers are generally used in composite materials having an epoxy matrix resin. In this case, the fibers are sized with an uncured epoxy resin and are cut to given lengths for use as a material to reinforce the matrix resin. When the fibers sized with an uncured epoxy resin are used with a thermoplastic matrix resin the sized carbon fibers contacts a heated thermoplastic resin (heating may be necessary to dry the resin) in the hopper, the epoxy resin softens and the fibers are prone to open. This also makes the stable production of a uniform resin composition difficult.